The beneficial and deleterious effects of ionizing radiations have been recognized for more than 80 years. In particular, megavoltage beams of x-rays and electrons from linear accelerators have found increasing use in cancer radiotherapy over the last twenty years. However, our knowledge about the fundamental mechanisms of radiation-induced reactions in tissue-like materials is still not totally understood for these beams. The proposed project is intended to provide a comprehensive set of data on the macroscopic and microscopic physical parameters of x-ray and electron therapy beams from a Clinac 18 linear accelerator. In addition, measurements of the biological effectiveness of these beams on simple biological systems will be made. Physical measurements of energy fluence spectra and microdosimetric spectra for beams of 10-MeV bremsstrahlung x-rays and 6- to 18 MeV electrons will be made, using a variety of nuclear radiation detectors, including a solid state GeLi detector, NaI(TI) scintillators, and a gas proportional chamber (i.e., a Rossi type linear energy transfer counter). Biological measurements of relative biological effectiveness (RBE) and oxygen enhancement ratio (OER) for the same beams will be made on cultures of monolayer cells and multicellular tumor spheroids. These data will provide information on the correlations between the physical and biological properties of typical beams used in radiotherapy. The predictions of various theorectical models relating these effects can be tested using these data as input parameters. This should improve our understanding of the fundamental processes involved in radiation-induced injury in biological systems.